In U.S. Pat. No. 4,364,035 a mouse is disclosed movable over a two-color grid of lines on a reflective surface. The lines are made with optically transmissive inks and are illuminated by a dual color monochromatic light source, such as a two-color LED, which rapidly and continuously switches from one color to the other. The colors of the light source are selected to match the spectral transmissivity of the two-color grid on the surface. Typically a set of parallel, equally spaced lines would have one color and an orthogonal set of parallel, equally spaced lines would have another second color with white or metallic reflective surfaces between lines. An illuminated surface area is imaged onto a light detector, within a housing movable over the surface. During illumination by light of one color, the detector distinguishes lines only of the other color, which appear dark against the reflective field. As the focal area within the housing crosses lines, the detector senses changes in contrast ratio and generates electrical signals representing the line crossings. If the lines are arranged in a grid, line crossings of one color may represent horizontal distance, while line crossings of the other color represent horizontal distance. The crossings may be counted by X and Y registers for controlling a cursor.
The same patent describes an alternate embodiment of the invention wherein two sources and a plurality of detector cells are provided. Instead of switching colors in a dual color source, a first pair of detector cells is sensitive to one color, while another pair of detector cells is sensitive to the opposite color. For example, two cells may be aligned for horizontal right and left detection, while two perpendicular cells may be aligned for vertical up and down motion detection. Unlike the previously described embodiment, this embodiment requires no clocking of the source and detector.
In the aforementioned patent, it was necessary to take the detected signals and compare them against a common reference signal from a voltage source, known as a threshold voltage. The threshold voltage might be set by a microprocessor. Disadvantages of this absolute threshold technique include inability to compensate for wide component variation, low signal-to-noise ratio and difficulty in establishing the threshold.
An object of the invention was to eliminate the use of the threshold signal and simplify electrical circuits required to read detector cells. Another object was to obviate the need for highly reflective lines by improving the signal-to-noise ratio of detection circuits for an optical mouse.